A flash memory can store information in accordance with a threshold voltage changed by injecting or discharging electrons to/from a floating gate or the like of a memory cell transistor. In the specification, a state where the threshold voltage of the memory cell transistor is low will be called an erase state, and a state where the threshold voltage is high will be called a write state. In the case of storing information in accordance with write data, a high voltage is applied to a memory cell transistor in the erase state in accordance with a logic value of the write data. To obtain a desired threshold voltage in the memory cell transistor, relatively long processing time is required.
There is a conventional flash memory card on which a flash memory chip and a memory controller are mounted and which employs an interleave writing operation to achieve seemingly high speed of the writing operation. The interleave writing operation is performed in such a manner that a plurality of flash memory chips are mounted on a card board, one of the flash memory chips is instructed to perform a writing operation and, after that, another flash memory is instructed to perform a writing operation and starts the writing operation. To make writing operation time unseen by the operation, a number of flash memory chips have to be mounted. Specifically, when write setup time for supplying a write address and write data to one flash memory chip and giving an instruction to perform the writing operation is compared with time of the writing operation of writing the write data into the memory address instructed by the write setup, the writing operation time is much longer. If the write setup is sequentially made to the other flash memories so as to overlap the writing operation time, the writing operations on most of the flash memory chips can be performed partially in parallel, so that the writing operation time of many of the flash memory chips is unseen.
However, in the conventional method of performing interleave writing on the flash memory unit basis, a number of flash memory chips have to be mounted to make the writing operation time unseen, so that the size and cost of the memory card increases.
An object of the invention is to provide a memory system and, further, a memory card capable of realizing higher write speed without mounting flash memory chips of the number which is large to an extent that the size of the memory card increases or the cost increases.
The above and other objects and novel features of the invention will become apparent from the description of the specification and the appended drawings.